Shape memory alloys (SMAs) are metallic alloys that may recover apparent permanent strains when they are heated above a certain temperature. SMAs have two stable states or phases; a hot or austenite state and a cold or martensite state. The temperatures at which the SMA changes states (i.e. its crystallographic structure) are a characteristic of the particular alloy. Selecting the material composition of the alloy and anneal temperatures of the alloy may be used to control the alloy's transition temperatures.
In the austenite state, the alloy is hard and rigid, while in the martensite state, the alloy is softer and flexible. In the martensite state, the SMA may be stretched or deformed by an external force. Upon heating, the SMA will return to its austenite state and contract or recover any reasonable stretch that was imposed on it. Thus, the SMA recovers with more force than was required to stretch it out. This exerted force upon contraction may be used to perform any number of tasks such as, but not limited to, turning a device on or off, opening or closing an object, or actuating a device or object.